Page:Proceedings of the Royal Society of London Vol 60.djvu/185

170 where a + b is the sum of breadths of strip and space in the grating, i = angle of incidence, θ = angle of diffraction. The sign of θ is taken positive when it lies on the same side of the normal as the incident radiation.

In the above equation there are two interesting cases:-

(1) When the receiver is placed at C, θ = 0° 1=(a + b)sin i = nλ. (2) When the deviation is minimum i = θ 2 (a + b)sin i = nλ.

Arrangement of the Apparatus.

The grating, which is cylindrical, is placed vertically on a wooden table, with its centre at C, occupied in the diagram by the spiral spring coherer S. With the radius, which joins the centre to the apex of the grating, as a diameter, a circle is engraved on the table—the focal curve—on which the radiator and the receiver are always kept. A pin is ﬁxed immediately below the apex, and a graduated ring sunk in the table with this pin as the centre. The graduated

circle is used for the measurement of the angles of incidence and diffraction. Two radial arms revolving round the pin carry the radiator and the receiver. The ends of the arms near the pin have